The enzyme peroxidase, especially horseradish peroxidase, has many uses, and of particular importance is its use for labelling of immunological reaction partners, for example, haptens, antigens or antibodies, utilized in carrying out immunological test procedures. Peroxidase is preferably used in immunological test methods since its activity or presence can be readily and simply detected. For this reason, an immunologically-active material to which the peroxidase is bound is an essential ingredient in many commercially available kits used for carrying out enzyme-immune test procedures. Since, before use, such kits are transported over large distances and are stored for various periods of time, it is essential that the activity of the enzyme be preserved for as long a period of time as possible. However, it is known that the enzyme peroxidase is not very stable irrespective of whether it is present in isolated form or is bound to another component, particularly in low concentrations. The storage stability is therefore also low, which detracts from the commercial importance of such kits.
At present, peroxidase conjugates are stored in a reaction medium containing serum or serum protein, which can contain, for example, approximately 20% goat serum. In this connection, it has been shown that the presence of the serum increases the stability of the conjugate. On the other hand, with high storage temperatures (e.g. 37.degree. C.) and/or after a long storage period (e.g. 6 months) it has also been observed that the serum contributes to the inactivation of the peroxidase by cleaving the haemin part from the enzyme. This inactivation of the peroxidase is evidently caused by the haemin interactions between the peroxidase and the haemin-binding proteins of the serum.
In accordance with the present invention, a method is disclosed for reducing the inactivation of peroxidase in a medium containing serum or serum protein and thereby substantially improving the stability of said peroxidase.